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Physics Colloquium: BICEP2 Results

Thursday, May 8, 2014

3:30 PM-4:30 PM

Bruce Partridge, PhD, professor, Haverford College

At very early times in the history of the Universe (say 10-35sec), we expect microscopic quantum fluctuations in both density (scalar fluctuations) and in the fabric of space-time (tensor fluctuations, that is gravitational waves). If -- and only if -- the Universe later underwent a brief period of exponential expansion, known inflation, the scale or wavelength of these gravitational waves (GW) is expanded from microscopic to astronomical size. The presence of these highly "inflated" GW would be the most convincing evidence we could have that inflation did occur -- a smoking gun for inflation.

In turn, the easiest way to detect these GW is through their effect on the cosmic microwave background. Inflated GW introduce a characteristic and hard to mimic pattern of polarization in images of the cosmic microwave background on scales of a few degrees. These are known as B modes. The amplitude (as opposed to the scale) of these B mode signals depends on the ratio (r) of tensor to scalar fluctuations. The quantity r depends on which model (out of very many) of inflation one chooses. Values of r ranging from essentially zero to one are possible; values of r ~0.1 were expected from the simplest models of inflation.

The announcement by the BICEP Team a few weeks ago that they had detected B modes with amplitude r = 0.2 thus both offers strong proof that inflation did happen, and raises some questions about how inflation operated.

In this talk, I'll explain the background physics, then focus on two other issues: the tension between the BICEP result and earlier constraints on r, and what a measured value of r tells us about the mechanism of inflation.